Democrats keep insisting that the presidential race will be a choice, not just a referendum on the incumbent. And the Republican nominee, no matter who it is, will not be without flaws. As Maryland Gov. Martin O’Malley (D) put it at a Monitor-hosted breakfast Thursday, Obama is “not running against the Almighty, he’s running against the alternative.”

As of now, the GOP race seems to have boiled down to a choice between Texas Gov. Rick Perry and former Massachusetts Gov. Mitt Romney. That, of course, could change, especially if a major new prospect enters the race. But for the sake of argument, let’s say either Governor Perry or Mr. Romney will get the nod.

Polls show that GOP voters believe Perry is electable, but polls of general election voters show Romney faring better than Perry against Obama. The Real Clear Politics average gives Obama a four-point lead over Perry but just a one-point lead over Romney.

Among independent voters, Romney has the clear advantage. In the latest survey by the Democratic firm Public Policy Polling (PPP), Perry’s favorability with independents is just 23 percent (with 51 percent seeing him unfavorably). Romney is seen favorably by 44 percent of independents, and unfavorably by 39 percent.

Perry’s comments on Social Security appeared to hurt his standing in the PPP poll. He has repeatedly called the program a Ponzi scheme, without making it clear that his intent is to fix it, rather than do away with it. In recent days, he has attempted to clarify his position – that he would not eliminate Social Security.

Still, Perry remains ahead of Romney in national polls of Republican voters by double digits. Conservative tea partyers are far more enthusiastic about Perry than Romney, and are more energetic in their political involvement than mainstream Republicans. They could help Perry to the nomination.

But if that happens, Obama’s Democratic base – more worried about a Perry presidency than a Romney presidency – would likely coalesce and rise up to defend him.

Romney would be harder for Obama to defeat, says William Galston, a Brookings Institution scholar who served in the Clinton White House. “One can criticize Mitt Romney, but it’s not so easy to demonize him,” he says.

“In Stage One, the American people reached a rough-and-ready conclusion that they were willing to replace President Carter if a reasonable alternative presented itself,” Galston says. “In Stage Two, which didn’t crystallize till late in the campaign, till after the Reagan-Carter debate, they decided Reagan was a reasonable alternative or at least reasonable enough.”

The question is – if Perry gets the nomination – can he make himself seem “reasonable enough” to general election voters to defeat Obama? It’s still early, and most Americans aren’t yet paying close attention to the presidential contest. But when they do, if the country is in tougher shape than it is now, Perry might seem presidential enough. If the picture is murkier, and Obama is seen as marginally viable, then facing Perry (and not Romney) might be enough to hand him four more years.